Two case reports suggest that calcium carbonate interferes with the body's absorption of thyroid hormone when both were taken at the same time.
1,2

A prospective cohort study has validated these case reports.
3
Twenty individuals with hypothyroidism stabilized on long-term levothyroxine therapy were included in the trial. Participants were given calcium carbonate (1,200 mg daily of elemental calcium) for 3 months. During the period the calcium supplement was taken, thyroid hormone blood levels declined. But after calcium supplementation was stopped, thyroid levels climbed back up, slightly surpassing the levels measured at the beginning of the study.

It is thought that calcium combines with thyroid hormone, thus reducing its absorption.

To prevent this interaction, take thyroid hormone and calcium supplements as far apart as possible.

Iron salts (including ferrous fumarate, ferrous gluconate, ferrous sulfate, and iron polysaccharide) may impair the effect of the thyroid hormone levothyroxine, probably by forming a complex with it and decreasing its absorption.
4

To prevent a problem, take iron supplements and thyroid hormones as far apart as possible.

Soy formula may interfere with the absorption of thyroid medication in infants.
5
In addition, soy may directly interfere with thyroid function.
6,7
The result may be a need to increase the infant's dosage of thyroid medication. However, if you stop giving an infant soy formula, the thyroid dosage may need to be decreased. Of course, all changes relating to thyroid treatment should be managed by a physician.

Based on these findings, individuals with impaired thyroid function should use soy (such as, soybeans, soy milk, tofu) with caution.

Individuals with an enlarged thyroid gland are sometimes given high doses of thyroid medication to shrink it. However, this treatment can cause unpleasant side effects, including bone loss, heart palpitations, and a feeling of malaise. A double-blind trial suggests that the supplement L-carnitine may safely reduce the adverse effects of this treatment.
8

Revision Information

This content is reviewed regularly and is updated when new and relevant evidence is made available. This information is neither intended nor implied to be a substitute for professional medical advice. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with questions regarding a medical condition.